Electrochemical nanostructuring method and device

ABSTRACT

The present invention relates to a method for producing surface-structured substrates, wherein a predetermined defined surface structuring is transferred to the surface of a substrate by activating the substrate surface using at least one fine probe that interacts with the substrate surface, and material of a liquid, solid, or molten salt electrolyte is electrochemically deposited in a selective manner on the regions of the substrate surface that are activated thereby. The invention also relates to a device for surface structuring of substrates.

DESCRIPTION

[0001] The present invention relates to a method for producing surface-structured substrates, wherein a predetermined defined surface structuring or surface structure is transferred to the surface of a substrate by activating the substrate surface using at least one fine probe that interacts with the substrate surface, and material of a liquid, solid, or molten salt electrolyte is electrochemically deposited in a selective manner on the regions of the substrate surface which are thus activated. The invention also relates to a device for surface structuring of substrates.

[0002] The creation of complex, defined nanostructures at predetermined defined locations plays an increasingly important role in the area of microelectronics and submicroelectronics and in the area of nanotechnology. This is particularly true for the controlled production of printed conductors or so-called nanodevices having dimensions down to the nanometer range.

[0003] The use of probes, for example in the form of scanning tunnel microscopes, has allowed substances on the atomic scale to be manipulated in a targeted manner (for example, M. Crommie et al., Science 262, 218 (1993)). Numerous monographs and specialized books provide an overview of the diverse applications of such probes in the area of nanostructures (for example, R. Wiesendanger: Scanning Probe Microscopy and Spectroscopy, Cambridge University Press, Cambridge 1994). Many methods described in the literature enable the production of metallic and nonmetallic nanostructures by removal of material (for example, B. Irmer et al., Appl. Phys. Left. 73, 2051 (1998)). However, it is often desirable to not only remove material, but also to locally deposit metal in a targeted manner.

[0004] It is possible to perform such deposition using controlled electrochemical processes on the nanometer scale. Electrochemical methods on the nanometer scale offer the additional advantage of reversibility. Metallic structures may be deposited, etched, modified, and resolved as often as desired. In this respect, the electrochemical deposition of metals with sufficient localized resolution represents a considerable technical challenge. The importance of this area is underscored by the fact that electrochemical methods for the deposition of miniaturized printed conductors on integrated circuits have recently come into use. However, a problem arises in that for applications in nanoelectronics, structures defined on the nanometer scale must be produced by localized electroplating, but conventional methods are not able to provide the corresponding lateral resolution.

[0005] The use of scanning probe techniques is suitable for such electrochemical nanostructuring. Previous attempts have been limited for the most part to deposition using the tip of a scanning tunnel microscope (STM). Since it is usually not possible to deposit complicated structures on the nanometer scale directly onto the sample surface in a reproducible manner using electrochemical STM, a method was developed in which metal islands are first deposited on the STM tip and then mechanically transferred to the sample surface (D. M. Kolb et al., Electrochimica Acta 43, 2751 (1998)). However, the inadequate adhesion of the metal particles deposited on the sample surface represents a significant problem.

[0006] The object of the present invention, therefore, is to provide an electrochemical method and a device for carrying out the method which allow the defined creation of complex nanostructures by depositing metals on the sample surface in a targeted manner.

[0007] This object is achieved by the embodiments characterized in the claims.

[0008] In particular, a method for producing surface-structured substrates is provided in which a predetermined defined surface structuring is transferred, imaged, or written to the surface of a substrate by activating the substrate surface using at least one fine probe that interacts with the substrate surface, and material of a liquid, solid, or molten salt electrolyte is electrochemically deposited in a selective manner on the regions of the substrate surface which are thus activated. Within the scope of the present invention, “surface structuring” is understood to mean the deposition of defined shapes, such as nanodots or nanotubes in the form of metal islands or metal bridges, onto a specified substrate; that is, the decoration of a substrate. Within the scope of the present invention, “activation” is understood to mean a mechanical alteration of a surface of a substrate or of an adsorbate layer, chemisorbate layer, contamination layer, or passivation layer present on a substrate. As a result of this mechanically induced alteration of the surface or layer, deposition from an electrolyte occurs under an electrochemical voltage at which, without this mechanical alteration, deposition from an electrolyte would not take place. This mechanical alteration can occur before or during deposition. Thus, the choice of a suitable electrochemical voltage results in selective deposition at the site of the mechanical activation. The mechanical alteration is achieved by mechanical interaction or mechanical contact with a structuring tool, which within the scope of the present invention is referred to as a “probe.”

[0009] In particular, the tip of a scanning probe microscope may be used as a probe. In the method according to the invention, the tip of a scanning force microscope, a scanning tunnel microscope, an optical near-field microscope, a scanning ion conductivity microscope, or a scanning electrochemical microscope is preferably used as a probe. The tip is usually mounted on a microfabricated leaf spring, also known as a cantilever. The probe may be in continuous or intermittent contact with the substrate surface. For example, the probe may slowly trace the shape of the structure to be deposited on the surface while moving back and forth parallel to the surface in any desired direction with respect to the contour line traced by the probe, with a small amplitude, preferably 0.1 nm to 100 nm, particularly preferably 0.5 nm to 50 nm, and at a sufficiently high frequency, preferably 1 Hz to 500 kHz, particularly preferably 1 kHz to 150 kHz, in each case depending on the substrate to be structured and the cantilever used. Alternatively, the probe may move back and forth parallel to the surface along the contour line traced by the probe, with a small amplitude and at a sufficiently high frequency, in two mutually orthogonal directions, for example by revolving. The amplitude movement of the localized probe may occur periodically, for example sinusoidially. Within the scope of the present invention, the amplitude movement of the probe is usually controlled by computer. The probe or tip may, for example, be periodically moved back and forth in the desired structuring direction while at the same time a slow advance of, for example, 0.1 μm to 100 μm, preferably 0.5 μm to 50 μm, continuously moves the probe along the structure to be generated. The back and forth movement as well as the advance preferably take place in the plane of the substrate surface to be structured. Each of the two partial movements may be controlled by computer.

[0010] In one embodiment of the method according to the invention, the selective deposition can be carried out by applying an electrical voltage of preferably −2 mV to −10 V between the substrate surface and another electrode, without applying a voltage to the probe used for the activation. The probe can be wired to zero voltage relative to the substrate surface, or not wired at all (open circuit). In a further embodiment of the method according to the invention, the selective deposition is carried out by applying an electrical voltage between the substrate surface and another electrode and by also applying a voltage to the probe used for the activation. In a further embodiment of the method according to the invention, the selective deposition is carried out by applying a voltage between the substrate surface and the probe used for localized activation. Finally, the selective deposition can also be carried out by electroless deposition.

[0011] Based on a predetermined defined substrate surface, or a pattern transferred or written to the substrate, lateral forces of for example 10 nN to 10 μN, that is, forces acting tangentially or substantially tangentially to the surface, such as frictional forces, for example, between the probe and the substrate surface may be used for the activation or modification of the substrate surface. In addition, normal forces between the probe and the substrate surface, that is, forces acting perpendicularly or obliquely to the surface, may be used for the activation. Furthermore, the activation may also occur by creating atomic or microscopic defects in the range of, for example, 0.1 nm to 100 nm on the substrate surface. In a particular embodiment of the method according to the invention, the substrate surface is covered by an organic adsorbate layer, a self-assembled monolayer, an inorganic- or organic-based polymer film, such as siloxane-based or polyolefin-based, or an inorganic or organic film. In this case, the activation of the substrate surface can be carried out by localized impairment or removal of this film or layer, or by creating defects in this film or layer.

[0012] In the method according to the invention, the activation may be carried out continuously or discontinuously during the electrochemical deposition. Alternatively, first the localized activation may be carried out continuously or discontinuously and then electrochemical deposition is carried out by applying a voltage. In addition, the localized deposition may be carried out during as well as after the activation, this method allowing the option of a targeted redevelopment, that is, targeted additional growth of the structure by further electrochemical deposition, even after conclusion of the activation process using the probe.

[0013] Within the scope of the method according to the invention, instead of a single probe an arrangement or array of two or more probes, for example up to 1,000 or more probes, may be used, resulting in a parallel, that is, simultaneous, structuring by more than one probe. Such an array of probes may be used, for example, for the parallel writing of identical structures. In addition, a master structure may be used as an array, so that the structure of the master determines the structure of the electrochemically created structure (electrochemical replication by induced activation).

[0014] The method according to the invention may also be carried out in such a way that first a predetermined defined structure is deposited on the substrate surface by applying a deposition voltage with simultaneous activation by the probe. Then, after the predetermined structure has been deposited, the applied electrochemical voltage is set at a holding voltage so that the previously created substrate surface or pattern neither resolves nor undergoes further growth. Next, at a simultaneously applied holding voltage the substrate surface is allowed to passivate, followed by the creation of a subsequent structure in the same manner, resulting in sequential deposition of different structures. Using such a process, it is possible to deposit a new (additional) structure by further structuring while applying a deposition voltage, without continued growth or alteration of the previously written structures. In this manner, any desired number of structures may be created in succession by repeating the process. In such an array of probes, such a sequential deposition of structures may also be used for parallel writing of identical structures.

[0015] Within the scope of the method according to the invention, solutions of copper salts, lead salts, silver salts, or gold salts, for example CuSO₄, PbSO₄, AgNO₃, AuCN, AuCl₃, and the like, may be used as electrolyte. In addition, monomer electrolytes such as pyrrole salts, or polymer electrolytes such as PPV or polypyrrole may be used. Furthermore, aluminum chloride-1-methyl-3-butylimidazolium chloride (MBICI) may be used as molten salt electrolyte. Aqueous copper solution is preferably used as electrolyte. The substrate to be structured or decorated according to the predetermined defined surface structuring or according to the predetermined pattern can be chosen from a monocrystalline or polycrystalline metal, or metal films or metal island films or alloys thereof that are vapor-deposited or produced by sputtering, or graphite, in particular HOPG [highly oriented pyrolytic graphite], ITO [indium tin oxide], semiconductors such as Si, GaAs and their doped modifications, and electrically conductive polymers such as doped polyacetylene. Gold is preferably used as the metal.

[0016] The method according to the invention is particularly suited for producing nanotubes and nanodots in the size range of for example 0.5 nm to 700 nm, preferably 2 nm to 100 nm, for structure width and 0.5 nm to 10,000 nm for structure length, for example in the form of metal islands. In addition, the method according to the invention can be advantageously used in microelectronics and nanoelectronics to produce so-called nanodevices, for the production of printed conductors on integrated circuits, or information units for data storage, for example.

[0017] In addition, according to the present invention a device is provided for surface structuring of substrates, comprising at least one fine probe which interacts with the substrate surface and which transfers a predetermined defined surface structuring to the surface of a substrate by activating the substrate surface, at least one electrolyte feed or electrolyte supply which furnishes a liquid, solid, or molten salt electrolyte for the electrochemical deposition of material in a selective manner on the regions of the substrate surface activated by the probe, a device for computerized tracing of a preprogrammed contour line or surface structure, and a device which simultaneouslycontrols a superimposed periodic or nonperiodic lateral movement of the probe, such as a sine generator which actuates the x, y actuating units such as piezoactuators, for example, which move the probe in a lateral direction relative to the sample or substrate surface, or a suitable software module with a computer interface (converter card, for example), and one or more amplifiers.

[0018] The figures show the following:

[0019]FIG. 1 shows the AFM [atomic force microscope] micrograph of two copper structures (“A P”) electrochemically deposited independently of one another from a CuSO₄ solution onto a gold film, the deposition taking place at a deposition voltage of −0.06 V and being locally activated using the AFM tip (scanning field 2.2 μm ×2.4 μm).

[0020]FIG. 2 shows the structure (a) entered into a computer and the corresponding AFM micrograph (b) of a copper structure (benzene ring) electrochemically deposited from a CuSO₄ solution onto a gold film, the deposition taking place at a deposition voltage of −0.05 V and being locally activated using the AFM tip (scanning field 1 μm ×1 μm).

[0021]FIGS. 3a-c show AFM micrographs which demonstrate the reversibility of tip-induced electrochemical structuring (Au/Cu²⁺ system), in which (a) a “6” was deposited, (b) this “6” was resolved, and (c) a “9” was deposited at the original site (the point-shaped defect in the upper left corner of the image (see arrow) was used for orientation; typical values for the deposited structures were approximately 15 nm; scanning field 1.6 μm ×1.6 μm).

[0022]FIG. 4 shows an example of a device according to a preferred embodiment of the invention, illustrating a preferred instrumentation setup for carrying out the method according to the invention: an electrochemical scanning force microscope (AFM) with integrated potentiostats by which the various voltages for deposition, erasure, and “holding” of structures may be controlled by computer. The working electrode is made of a gold film, for example. Present within the liquid cell is the electrolyte, for example 0.05 M H₂SO₄ and 1 mM CuSO₄ in aqueous solution. Copper wires 0.25 mm thick, for example, may be used as the reference electrode and counterelectrode.

[0023] In the method according to the invention, the sample surface or substrate surface is locally modified or activated using a fine probe. For material deposition, the value of the electrochemical voltage is chosen so that the deposition selectively takes place at the applied voltage only at the sites activated by the probe before or during the deposition process. FIG. 1 shows two independent copper nanostructures created by the method according to the invention, in the shape of an “A” and a “P,” deposited on a gold island film. Using the method according to the invention, any desired lateral structures may be deposited onto a substrate (electrochemical CNC [computer numerical controlled] machine), using a computer. The deposition is carried out in a targeted manner according to the method of the present invention at the positions which are activated by scanning with the probe, such as a scanning force microscope tip, for example, immediately before or during the deposition.

[0024]FIG. 2 elucidates the computerized method according to the present invention, based on a nanostructure produced in the shape of a benzene ring using the method according to the invention. The line width of the achievable structures is in the range of 10 nm to 60 nm, depending on the duration of deposition and the deposition current. First, the predetermined structure is defined by computer, followed by structuring, in which locally defined electrochemical reactions are mechanically initiated by the AFM tip. The structures may be produced by the inventive method within several seconds by computer control. The created structure may then be imaged with the same probe that was used for the localized activation of the substrate surface.

[0025] The method according to the invention offers the additional advantage of complete reversibility, for example in the deposition of copper from copper sulfate onto a gold surface. Written structures can be erased, that is, electrochemically resolved, by applying a resolution voltage. The substrate surface can then by “rewritten”; that is, structures may be redeposited on the surface, regardless of where such structures were previously deposited. FIG. 3 shows the reversibility of the method. The numeral “6” was first written at a site. The structure was then erased by applying a resolution voltage. The numeral “9” was then deposited at the same site. Within this scope, for example selective erasure of individual or multiple structures or parts of such structures may be carried out by mechanical action of the probe in order to activate or support the electrochemical resolution process in a site-selective manner. The created nanostructures can be passivated against resolution or partially passivated for example by a thin passivation layer such as thioles, organic films, polymer films, or oxides such as Al₂O₃, or resolution can be inhibited by these layers or films. Site-selective or localized resolution may be achieved by localized removal or impairment of this layer, or by creating defects within this layer, with the probe used according to the invention. The same probe can be used for the production or creation of structures as well as for the selective resolution of the structures. The resolution is preferably carried out by applying an electrical voltage of preferably +2 mV to +10 V to the probe.

[0026] The present invention is described in more detail with reference to the following examples, without being limited to these examples.

EXAMPLE 1

[0027] Deposition of independent nanostructures of copper onto gold was carried out. Deposition was performed using an electrolyte of 0.05 M H₂SO₄ and 1 mM CuSO₄ in aqueous solution under ambient conditions at room temperature. A vapor-deposited gold island film 50 nm thick was used as the substrate and working electrode, and a copper wire 0.25 mm thick was used as the reference electrode and counterelectrode. A scanning force microscope tip (cantilever) was used as a localized probe. First, copper was deposited onto the gold island film by tracing the outline “A” 50 times at a speed of 8 μm/sec and a deposition voltage of −0.06 V between the working electrode and the reference electrode. After a pause of 10 seconds, the outline “P” was similarly deposited at a nearby site. A holding voltage of −0.03 V was applied, and after a pause of 10 seconds the surface was imaged using the scanning force microscope. FIG. 1 shows the scanning force microscope micrograph of a structure thus produced.

EXAMPLE 2

[0028] The deposition of an isolated fine structure was carried out according to the inventive method under ambient conditions at room temperature, as follows:

[0029] The electrolyte was 0.05 M H₂SO₄ and 1 mM CuSO₄ in aqueous solution with a pH of about 1.3. A vapor-deposited gold island film 50 nm thick was used as the substrate and working electrode, and a copper wire 0.25 mm thick was used as the reference electrode and counterelectrode. A scanning force microscope tip (cantilever) was used as a localized probe. It has proved to be advantageous to use conductive cantilevers (in this case coated with titanium oxide). FIG. 4 shows an example of the instrumentation setup used, in which a scanning force microscope was combined with an electrochemical liquid cell together with potentiostatic control. For deposition a voltage of −0.05 V was applied between the working electrode and the reference electrode, this voltage being chosen so that no additional three-dimensional copper deposition occurred. A pressure force of at least 4×10⁻⁸ N was required for structuring. The structure was deposited by tracing a structure, which had previously been entered into a computer (see benzene ring, FIG. 2a), 20 times at a speed of 8 μm/sec. The corners of the hexagon were reinforced in a targeted manner by writing a small circle (radius 15 nm). Further resolution or growth of the deposited structure was prevented by applying a holding voltage of −0.03 V between the working electrode and the reference electrode. After 20 seconds a scanriing force microscope image was recorded (FIG. 2b), which shows the nanostructure produced according to the inventive method with a line width of approximately 15 nm.

EXAMPLE 3

[0030] Deposition was performed using an electrolyte of 0.05 M H₂SO₄ and 1 mM CuSO₄ in aqueous solution under ambient conditions at room temperature. A vapor-deposited gold island film 50 nm thick was used as the substrate and working electrode, and a copper wire 0.25 mm thick was used as the reference electrode and counterelectrode. A scanning force microscope tip (cantilever) was used as a localized probe. First, copper was deposited by tracing the outline “6” 13 times at a speed of 8 μm/sec and a deposition voltage of −0.06 V between the working electrode and a reference electrode. After a pause of 10 seconds and application of a holding voltage of −0.03 V, the scanning force microscope image was obtained (FIG. 3a). The structure was erased by applying a resolution voltage of 0.2 V for 3 minutes. After applying a holding voltage of −0.03 V it was possible to once again obtain the image using the scanning force microscope. FIG. 3b shows the surface from 3 a thus erased. The dot-shaped defect in the upper left corner of the image (see arrow) was used for orientation. No signs of the previously deposited structure are detectable on the surface. Similarly, the outline “9” was then deposited [by tracing] 13 times at a speed of 8 μm/sec and a deposition voltage of −0.06 V. After applying a holding voltage of −0.03 V and pausing for 10 seconds, the scanning force microscope image was obtained. FIG.3c shows the structure “9” created on the same surface region upon which the “6” was written and erased. No signs of any previous deposition are detectable. The method according to the invention erases the surface and allows further structuring in a reversible manner. This example demonstrates that the substrate surface is not irreversibly damaged by the tip-induced reaction or activation, but, rather, that the redeposition is completely independent of the previous history of the site. 

1. Method for producing surface-structured substrates, wherein a predetermined defined surface structuring is transferred to the surface of a substrate by activating the substrate surface using at least one fine probe that interacts with the substrate surface, and material of a liquid, solid, or molten salt electrolyte is electrochemically deposited in a selective manner on the regions of the substrate surface which are thus activated.
 2. Method according to claim 1, characterized in that the tip of a scanning probe microscope is used as a probe.
 3. Method according to claim 1 or 2, characterized in that the tip of a scanning force microscope, a scanning tunnel microscope, an optical near-field microscope, a scanning ion conductivity microscope, or a scanning electrochemical microscope is used as a probe.
 4. Method according to one of claims 1 through 3, characterized in that the selective deposition is carried out by applying a voltage between the substrate surface and another electrode, without applying a voltage to the probe used for the activation.
 5. Method according to one of claims 1 through 3, characterized in that the selective deposition is carried out by applying a voltage between the substrate surface and another electrode, and by also applying a voltage to the probe used for the activation.
 6. Method according to one of claims 1 through 3, characterized in that the selective deposition is carried out by applying a voltage between the substrate surface and the probe used for the activation.
 7. Method according to one of claims 1 through 3, characterized in that the selective deposition is carried out by electroless deposition.
 8. Method according to one of claims 1 through 7, characterized in that lateral forces between the probe and the substrate surface are used for the activation.
 9. Method according to one of claims 1 through 7, characterized in that normal forces between the probe and the substrate surface are used for the activation.
 10. Method according to one of claims 1 through 7, characterized in that the activation is carried out by creating atomic or microscopic defects in the range of 0.1 nm to 100 nm on the substrate surface.
 11. Method according to one of claims 1 through 7, characterized in that the substrate surface is covered by an organic or inorganic adsorbate layer, a self-assembled monolayer, a polymer film, or an inorganic or organic film, and the activation of the substrate surface is carried out by localized impairment or removal of this film or layer, or by creating defects in this film or layer.
 12. Method according to one of claims 1 through 11, characterized in that the activation is carried out continuously or discontinuously during the electrochemical deposition.
 13. Method according to one of claims 1 through 11, characterized in that first the activation is carried out continuously or discontinuously, and then electrochemical deposition is carried out by applying a voltage.
 14. Method according to one of claims 1 through 11, characterized in that the deposition is carried out during as well as after the activation.
 15. Method according to one of claims 1 through 14, characterized in that an arrangement of two or more probes is used.
 16. Method according to one of claims 1 through 15, characterized in that first a predetermined defined structure is deposited on the substrate surface by applying a deposition voltage with simultaneous activation by the probe, and after the predetermined structure has been deposited the applied electrochemical voltage is set at a holding voltage and the substrate surface is allowed to passivate, followed by the creation of a subsequent structure in the same manner, resulting in sequential deposition of different structures.
 17. Method according to one of claims 1 through 16, characterized in that solutions of copper salts, lead salts, silver salts, or gold salts are used as electrolyte.
 18. Method according to one of claims 1 through 17, characterized in that the substrate is chosen from a monocrystalline or polycrystalline metal, or metal films or metal island films or alloys thereof that are vapor-deposited or produced by sputtering, or graphite, ITO, semiconductors, or electrically conductive polymers.
 19. Device for surface structuring of substrates, comprising at least one fine probe which interacts with the substrate surface and which transfers a predetermined defined surface structuring to the surface of a substrate by activating the substrate surface, and at least one electrolyte supply which furnishes a liquid, solid, or molten salt electrolyte for the electrochemical deposition of material in a selective manner on the regions of the substrate surface activated by the probe, a device for computerized tracing of a preprogrammed contour line, and a device which simultaneously controls a superimposed periodic or nonperiodic lateral movement of the probe. 